This invention relates to radio receivers, and more specifically relates to automatic gain control techniques for such receivers.
Radio frequency (RF) and intermediate frequency (IF) automatic gain control (AGC) using quasi-logarithmic amplifiers controlled in hardware or software loops are commonly used in radios, including aviation radio receivers. Although such techniques have been useful in the past, they are inadequate to meet the new standards for aviation radios recently enacted. This invention addresses that need and provides a solution.
The preferred embodiment is useful in a radio receiver in order to achieve automatic gain control. In such an environment, a radio frequency signal is received, is converted to a first signal and is down converted to an intermediate frequency signal. The intermediate frequency signal is converted to a second signal and also is down converted to a baseband signal. A first control signal is generated in response to the first signal and a second control signal is generated in response to the second signal. The first control signal limits the radio frequency signal to a first predetermined value, and the second control signal limits the intermediate frequency signal to a second predetermined value. Likewise, the baseband signal is limited to a third predetermined value.
By using the foregoing techniques, the gain of a radio receiver automatically can be controlled with a degree of accuracy previously unknown. Time constants may be set with increased flexibility, and interactions between the RF, IF and baseband AGC zones can be reduced. For radio frequency signals in the range of 118-137 MHz, the preferred embodiment can attenuate strong interferences within two MHz. For intermediate frequency signals of approximately 121.4 MHz, the preferred embodiment can be used to properly load an A-D converter over a 40 decibel dynamic range. Similarly, at baseband frequencies, at least 65 decibels of dynamic range over and above the radio frequency and intermediate frequency AGC can be provided.
According to another embodiment of the invention, by using digital processing, an AGC can be implemented in floating point arithmetic with unlimited dynamic range.